Synthesis and properties of anhydrous rare-earth phosphates, monazite and xenotime: a review

The synthesis methods, crystal structures, and properties of anhydrous monazite and xenotime (REPO4) crystalline materials are summarized within this review. For both monazite and xenotime, currently available Inorganic Crystal Structure Database data were used to study the effects of incorporating different RE cations on the unit cell parameters, cell volumes, densities, and bond lengths. Domains of monazite-type and xenotime-type structures and other AXO4 compounds (A = RE; X = P, As, V) are discussed with respect to cation sizes. Reported chemical and radiation durabilities are summarized. Different synthesis conditions and chemicals used for single crystals and polycrystalline powders, as well as first-principles calculations of the structures and thermophysical properties of these minerals are also provided.


Introduction
Monazite and xenotime compounds are anhydrous rare-earth (RE) phosphates with the chemical formula REPO 4 but with different crystal symmetries.The mineral name "monazite" is derived from "monazein" meaning "to be solitary" in Greek, and the mineral name "xenotime" is derived from the combined words of "xenos" and "time" meaning "foreign" and "honor" in Greek. 1 Naturally occurring monazite and xenotime minerals are oen found as accessory minerals along with allanite, sphene, uorite, and apatite in granitic rocks, pegmatites, carbonatites, and gneisses. 1,24][5] For example, the compositions of natural monazites from seven different regions were 35-65 mass% of mixed REs, 24-30 mass% of P 2 O 5 , 0-16 mass% of UO 2 , 4-14 mass% of ThO 2 , and 2-7 mass% of other oxides, and among the mixed REs, La, Ce, and Nd were the major components. 2,3,6The natural xenotime mineral generally contains mixed heavy REs (i.e., RE = Gd / Lu + Y, Sc) and less actinides compared to monazite (i.e., RE = La / Dy).The compositions of natural xenotime from two regions were 62-65 mass% of mixed RE oxides, 29-36 mass% of P 2 O 5 , 0.1-1.6 mass% of UO 2 , 0.2-0.3mass% of ThO 2 , and 0.1-1.3mass% of other oxides, and among the mixed REs, Y, Gd, Dy, and Er were the major components. 7,8habdophane minerals are hydrated RE-phosphates with the chemical formula of REPO 4 $xH 2 O and they are found as accessory minerals in granitic rocks.Rhabdophane minerals generally contains mixed REs with relatively high amounts of Ce and La elements, U and Th actinides, and one water molecule (7-8 mass%) per formula unit. 9Studies have shown that the rhabdophane structure can transform to the monoclinic monazite structure through heat treatments. 10,11[15]17,29 The exibility of the RE-O bond distances in REO x polyhedra, while maintaining the structure, enables the incorporation and substitution of different heavy radioactive cations.This possibility of monazite and xenotime to incorporate many different radionuclides into a single structure makes it an ideal waste form that eliminates the need to determine the radionuclide partition coefficients between phases and concerns for differences in mechanical properties and corrosion mechanisms. 12Both monazite and xenotime can be synthesized relatively easily (see Section 2 for details), and their chemical durabilities are much higher (up to a factor of 20 in leach rates for certain elements) than borosilicate glass waste forms. 12Natural monazite was shown to maintain high retention of ssion products during erosion, which was attributed to its slow dissolution rate. 13In addition, both compounds have high thermal stabilities with melting temperatures above 2000 °C. 122][23] Wang et al. 21investigated the luminescence behavior of Eu-doped LaPO 4 nanorods and showed the emission spectrum dependency on polarization for electric and magnetic dipole transitions.With high quantum efficiency and a high degree of polarization, the Eu-doped LaPO 4 nanorods can be used as high resolution probes in 3D ow-shear tomography. 21Hashimoto et al. 22 observed that doping a small amount of Th and borate into monazite compounds containing mixed RE cations of La, Ce, and/or Tb improved the light emission intensity and stability of green light, and these phosphors used in uorescent lamps can minimize the brightness loss at high temperatures.Jeon et al. 23 doped La-and Ca-containing monazite compounds with Eu and/or Dy ions, and single-phase white-light emission was observed at near-ultraviolet excitation.
7][28] Morgan et al. 27 showed that LaPO 4 and alumina interfaces without impurities were stable and retained the ability to debond aer heat treatment at 1600 °C in air but observed the formation of La-containing b-alumina-magnetoplumbite in the presence of alkali metal or divalent elements near the interface.Kuo and Kriven 26 showed that three laminates composed of LaPO 4 as one component and Al 2 O 3 , Y 3 Al 5 O 12 , or LaAl 11 O 18 as the other component were thermally stable up to 1600 °C and could be used as high-temperature materials in an oxidizing environment.Boakye et al. 28 showed that coating the SiC bers with LaPO 4 did not degrade ber strength, and a heat treatment at 1200 °C for 1-20 h in argon resulted in the formation of La 2 Si 2 O 7 , while the monazite was stable with SiC when the same heat treatment was done in air.
In this review, synthesis methods, crystal structures, and properties of anhydrous REPO 4 compounds are summarized.Studies on the chemical durability and radiation stability, as well as different models for phase stability of monazite and xenotime are also summarized.

Synthesis methods
Different methods, including ux-assisted, solid state, hydrothermal, aqueous, dehydration, and gel-based methods, have been used for the synthesis of monazite and xenotime compounds.Table 1 summarizes the synthesis conditions for REPO 4 compounds.
Feigelson 30 synthesized single crystals of LaPO 4 , CePO 4 , PrPO 4 , NdPO 4 , SmPO 4 , EuPO 4 , and GdPO 4 monazite compounds as well as TbPO 4 and YbPO 4 xenotime compounds using Pb 2 P 2 O 7 ux.The mixture of RE oxides and lead hydrogen phosphate (PbHPO 4 ) with the mass ratio of 1 : 24 was placed in the Pt crucible and heated to 1300 °C at 300 °C h −1 and dwelled for 12 h at 1300 °C, and PbHPO 4 was converted to lead pyrophosphate (Pb 2 P 2 O 7 ) on heating.The mixture was slowly cooled to 975 °C at <4 °C h −1 and then naturally cooled to room temperature as the authors believed that crystallization did not proceed below 975 °C.The single crystals of monazite compounds were separated from the ux using diluted HNO 3 solution.The monazite crystals had platelike morphologies, and the crystal sizes of LaPO 4 , PrPO 4 , and NdPO 4 were ∼6 mm × ∼3 mm × ∼0.5 mm (the crystal sizes of other monazites were not reported).Decreasing cooling rate increased the crystal sizes of xenotime compounds.For YbPO 4 , a plate crystal with dimensions of 45 mm × 25 mm × 0.25 mm was obtained when cooled at 0.5 °C h −1 .The single crystal or polycrystalline compound of TbPO 4 could be prepared using TbO (0.002 mol) was added to round bottomed ask containing 13.7 mL of 14.6 M H 3 PO 4 and stirred with magnetic stir for 1-24 h.The solution was diluted by adding 100 mL of water and reuxed at 130 °C for 2 h.The precipitate was ltered and washed with DIW.The sample was dried overnight and then heated to 80 °C for 1 h, and half of sample was heated at 200 °C for up to 48 h for complete dehydration.Sample heated at 80 and 200 °C were compared.The synthesized LaPO 4 monazite was a polycrystalline powder containing some rhabdophane, and the xenotime powders were pure LuPO 4 and YbPO 4 compounds.
Cao et al. 32 synthesized LaPO 4 and CePO 4 nanorods using LaNO 3 , CeNO 3 , and H 3 PO 4 precursors.Here, 1 M La(NO 3 ) 3 or 1 M Ce(NO 3 ) 3 and 0.7 M H 3 PO 4 were added to a solution of cetrimonium bromide in cyclohexane and n-pentanol.Solutions were mixed for 30 min and transferred to autoclaves.The autoclaves were heated at 140 °C for 12 h and cooled to room temperature.The crystals were washed with ethanol and DIW several times and dried in vacuum at room temperature.The obtained LaPO 4 and CePO 4 nanorods had about 5 mm lengths and 20-60 nm diameters.
Li and Ma 33 synthesized LaPO 4 :Eu crystals using La(NO 3 ) 3 -$6H 2 O and Eu(NO 3 ) 3 $5H 2 O with a molar ratio of 1 : 0.05 dissolved in DIW, and addition 5 mL of 1 M NaH 2 PO 4 aqueous solution was done while stirring.Here, HNO 3 or NaOH were added to adjust to a specic pH.The solution was put into an autoclave and heated at different temperature (120- with the mass ratio of 1 : 4 was placed in the Pt crucible and heated to 1020 °C for 24 h.A seed crystal was immersed in the solution and rotated at 30 rpm, and the single crystal was grown in the mixture cooling at 0.05-0.2°C h −1 for ∼30 d.The resulting single crystal had the size of several millimeters. Poitrasson et al. 46 synthesized polycrystalline NdPO 4 and GdPO 4 compounds using a combined method of gelation and ux-assisted growth.The NdPO 4 gel was prepared by dissolving Nd(NO 3 ) 3 and adding (NH 4 ) 2 HPO 4 solution.Precipitation of NdPO 4 occurred while drying for several days, and the nal NdPO 4 product was obtained aer briey heating above 600 °C

Crystal structures
Monazite crystallizes in the monoclinic P2 1 /n space group (REs are coordinated by nine oxygens), and xenotime crystallizes in the tetragonal I4 1 /amd space group (REs are coordinated by eight oxygens) and is isostructural to zircon (ZrSiO 4 ).Monazite compounds of lanthanides contain the lanthanide elements with larger ionic radii (r i ) including La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, and Dy whereas xenotime compounds contain the heavy lanthanide elements with smaller r i including Gd, Tb, Dy, Ho, Er, Tm, and Lu along with Sc and Y.The structural parameters of monazite and xenotime compounds at ambient conditions (293-300 K and 1 atm) reported in the Inorganic Crystal Structure Database (ICSD) are provided in Tables 2 and 3, respectively.It should be noted that, for the REPO 4 compounds with RE elements, the mid-range of lanthanides (i.e., Gd, Tb, Dy) can crystallize in both monazite and xenotime structures.Formation of monazite or xenotime for Gd, Tb, and Dy elements can be controlled by synthesis conditions (see Section 2 for details).Fig. 1 shows the relationship of monazite and xenotime structures with respect to the crystal radii (r c ) from Shannon. 49The unit cell parameters of the PmPO 4 compound have been reported, 50 but the overall structure data has not been reported in the ICSD.As for other RE elements, namely Y and Sc, YPO 4 and ScPO 4 (pretulite) have the xenotime structure.Both monazite and xenotime structures are commonly found in nonphosphate compounds including RE vanadates (REVO 4 ) and RE arsenates (REAsO 4 ). 1 Details of the monazite and xenotime structures are discussed in the following sections.
The nine oxygen atoms coordinating the REs in monazite include ve oxygen atoms forming a nearly equatorial pentagon whereas the other four oxygen atoms form a tetrahedron interpenetrating the pentagon (Fig. 2), and this coordination of REO 9 was described as a pentagonal interpenetrating tetrahedral polyhedron (PITP). 31,45The interpenetrating tetrahedron of REO 9 shares the edges with two adjacent PO 4 tetrahedra (Fig. 2a), resulting in a chain-like structure along the c axis (Fig. 2b).Fig. 2c shows the atomic arrangement of RE and P atoms projected down [001].
Distortion of REO 9 polyhedra and PO 4 tetrahedra in the monazite structure was reported in the literature. 1,59The monazite structure has four oxygen atom positions (i.e., O1, O2, O3, and O4), and the O2 atom is shared by three RE cations and a P cation whereas O1, O3, and O4 atoms are shared by only two RE cations and a P cation.This bonding results in REO 9 polyhedra with one longer RE-O2 distance compared to other eight RE-O distances.For example, the difference in RE-O bonding results in one longer ∼2.8 Å RE-O2 bond length compared to ∼2.5-2.6 Å of other eight RE-O bond lengths in CePO 4 . 59This RE-O coordination also affected the distortion of PO 4 tetrahedra with different P-O bond lengths and O-P-O bond angles within a given PO 4 tetrahedron. 59The distortion index 84 value (D) can be used to show the average deviation of RE-O bond distances from their means within the REO 9 polyhedra of monazite, and this is shown in eqn (1) where l i is the distance from the central atom (i.e., RE) to the ith coordinating atom, and l av is the average bond length.The distortion indices of REO 9 in monazite are in the range of 0.03-0.11and were relatively higher than the distortion indices (0.01-0.06) of REO 8 in the xenotime structures.The larger D value of REO x in monazite compared to xenotime was largely due to the one long RE-O2 bond.The distorted REO 9 polyhedron in monazite has a set of nine different RE-O bond lengths, and this coordination is correlated to its capability to incorporate various cations and polyoxoanions in the structure. 14Generally, this type of irregular coordination of metal ions does not induce severe symmetry, charge, or size constraints on the incorporated cation. 15,59 As mentioned earlier, the monazite structures incorporate light RE cations, including La / Dy.With larger RE cations (i.e., larger r c values) 49 in the crystal structure, the unit cell parameters (i.e., a, b, c) and volumes (V) increase linearly whereas the densities (r) decrease nonlinearly (Fig. 3).Fig. 3 was drawn using the average values of a, b, c, V, and r of monazite compounds at atmospheric conditions reported in ICSD for each given RE.The crystal structure data of PmPO 4 was not reported in the ICSD, and the unit cell parameter values from a study by Weigel et al. 50were used to t the trendline.
The RE-O bond distances of monazite compounds were compared, and larger cations in REO 9 polyhedra resulted in longer average RE-O bond distances (Fig. 4a).As discussed above, the RE cation in the monazite structure is coordinated by nine oxygen atoms, but presenting a single-digit coordination number might not be accurate in relatively distorted coordination polyhedra.The effective coordination numbers (CN eff ) [85][86][87] can be used to express more reasonable coordination numbers by accounting for all the surrounding atoms with a weighting scheme.The CN eff values of REO 9 were calculated using eqn (2) and (3) where w i is the bond weight of the i th bond, l av is dened in eqn (4), l i was dened above, and l min is the shortest bond distance in the coordination polyhedron.These equations were formulated by combining the concepts of Pauling with effective coordination numbers and mean ctive r i values. 87The average CN eff value calculated for each RE element in monazites is plotted in Fig. 4b, and REO 9 polyhedra with larger cations generally show higher CN eff values.
For the REO 8 coordination in xenotime, each oxygen atom is shared by two RE atoms and one P atom.Similar to monazite, the REO 8 polyhedra share the edges with two adjacent PO 4 tetrahedra (Fig. 2d), forming a chain-like structure along the c Table 2 Structural parameters of REPO 4 monazite compounds at ambient conditions (293-300 K and 1 atm) including unit cell parameters (a, b, c), b angle, cell volume (V), cell density (r), bond distances (i.e., RE-O, P-O), and the entry number for the Inorganic Crystal Structure Database (ICSD).The space group of listed compounds is P2 1 /n (SG# 14).Volume and density are calculated values, and RE-O and P-O are the average distances  4b).The unit cell parameters and volumes increase linearly with larger RE cations in the structures whereas the densities decrease nonlinearly (Fig. 5).
The RE 3+ cations at RE sites in the monazite and xenotime structures can be substituted by mixed +3 cations, a combination of +2 and +4 cations, or a combination of +2, +3, and +4 cations.Table 4 shows the list of synthetic monazite and xenotime compounds with mixed RE and summarizes the compositional effects.[90] Table 3 Structural parameters of REPO 4 xenotime compounds at ambient conditions (293-300 K and 1 atm) including unit cell parameters (a, b, c), cell volume (V), cell density (r), bond distances (i.e., RE-O, P-O), and the entry number for the Inorganic Crystal Structure Database (ICSD).The space group of listed compounds is I4 1 /amd (SG# 141).Volume and density are calculated values, and RE-O and P-O are the average distances  88 synthesized La 1−x Ce x PO 4 (x = 0-1) monazite compounds and showed that increasing La contents increased the unit cell parameters and cell volumes.Similarly, Terra et al. 89 observed that increasing La contents in La 1−x Gd x PO 4 (x = 0-1) monazites increased the unit cell parameters and cell volumes.Thust et al. 91 observed that increasing Eu content in La 1−x Eu x PO 4 (x = 0-1) monazite compounds increased the elastic stiffness coefficients, densities, heat capacities, and coefficients of thermal expansion (CTE).Arinicheva et al. 92 observed that microhardness, fracture toughness, unit cell parameters, and cell volumes decreased linearly with increasing Eu content in La 1−x Eu x PO 4 (x = 0-1) monazite compounds.Van Emden et al. 90 synthesized Nd 1−x Y x PO 4 (x = 0.05-0.3)monazite compounds using a solid state method at 1000 °C and showed that increasing Nd content increased the unit cell parameters and cell volumes.The same group also observed cocrystallization of both monazite and xenotime in Nd 1−x Y x PO 4 compounds when synthesized at 1200 °C. 90Hay et al. 93 synthesized the Gd 1−x Dy x PO 4 (x = 0-1) xenotime compounds and observed that pressure and/or shear stress can cause phase transformation of xenotime to monazite.Strzelecki et al. 94 investigated the thermodynamic properties of Er 1−x Yb x PO 4 (x = 0-1) xenotime compounds and observed that increasing Er content increased the enthalpies of formation and decreased the Gibbs free energies.The unit cell parameters and cell volumes of Er 1−x Yb x PO 4 (x = 0-1) increased with higher Er content as predicted by Vegard's law. 94Xiao et al. 95  (x = 0-1) and observed that these compounds with large differences in RE sizes resulted in the presence of both monazite and xenotime phases in the nal product, and the phase fractions were dependent on the corresponding RE contents.
For the monazites with a mixed combination of +2 and +4 cations at RE sites, Pb, Cd, or alkaline earth metals (e.g., Mg, Ca, Sr, Ba) can be incorporated with actinide cations (e.g., Th, U, Np), respectively.The chemical formula is (M )PO 4 and many examples have been documented, including Ca 0.5 -Th 0.5 PO 4 , 97 Mg 0.5 Th 0.5 PO 4 , 98 Sr 0.5 Th 0.5 PO 4 , 99 Pb 0.5 Th 0.5 PO 4 , 99 Ca 0.5 U 0.5 PO 4 , 100 Mg 0.5 U 0.5 PO 4 , 98 Sr 0.5 U 0.5 PO 4 , 98 98,99,104 The monovalent cations (e.g., Li, Na, K, Rb, Cs) with +3 RE cations or Ce 4+ can occupy the RE site of compounds that have similar structures to monazites [e.g., Na 3 La(PO 4 ) 2 , LiCe 2 (-PO 4 ) 3 , K 2 Ce(PO 4 ) 2 ]. 105,106 AXO 4 monazite-type structure stabilities can be dependent on various factors such as composition, temperature, pressure, and the irradiation conditions.Clavier et al. 14 reviewed crystal chemistry of the AXO 4 monazite-type compounds in terms of eld of stability versus composition, with all the substitution possibilities on the cationic and anionic sites leading to the monazite structure.
Several models, which include structure-eld maps (Fig. 6a) 107,108 and classication diagrams (see Fig. 6b for a modied Bastide diagram), [109][110][111][112] have been developed to correlate the stability of the monazite-type structure with geometric criteria.These representations provide opportunities to predict the structure of a compound.The challenge persists regarding the boundaries of the stability domain within this eld.Numerous studies have investigated the stability domain of the monazite-type structure, aiming to develop predictive models that could anticipate whether a compound might take on the monazite structure.Carron et al. 113 calculated that a value of 1.86 (ratio between the X-O bond length in the AXO 4 compounds and the r i , denoted as X/r i ) seems to specify the size limit of both the cation and the anion at the xenotime-monazite structural frontier.This ratio led to them 113 proposing the potential for anionic isomorphous substitution among REphosphates, RE-silicates, RE-arsenates, and RE-vanadates.Macey 114 further determined that monazite and zircon structures have X/r i < 1/1.56 and X/r i > 1/1.56, respectively.
Podor and Cuney 97 subsequently explored the stability range of the monazite structure, primarily concentrating on (M 1-2x 3+ )(M x 2+ )(M x 4+ )PO 4 compounds, where stability depends on three parameters (x, average r i , and the ratio of M elements).Nonetheless, discrepancies with experimental observations could still occur, particularly concerning miscibility gaps.Kolitsch and Holtstam 116 devised a visual representation outlining the stability regions of both monazite and zircon-type structures, relying on existing data for REXO 4 (X = P, As, V) compounds.While this diagram offers a broad view of the monazite structure stability eld, it is insufficient to correctly describe the partial solid solutions between two REXO 4 compounds, especially those involving a light RE element and a heavy RE element, and the presence of a miscibility gap. 14Properties

Mechanical and thermal properties
Monazite and xenotime compounds exhibit interesting mechanical and thermal properties across different compositions and structures.Table 5 summarizes the mechanical  by sample preparation and sintering conditions for the pellets. 117,118able 6 summarizes the thermal properties of monazite and xenotime including heat capacity (C p ), the CTE, thermal conductivity (k), thermal diffusivity (a), and melting temperatures (T m ).In general, the CTE values of monazite compounds seem to increase with smaller RE cations and RE-O distances in the structures.In general, the CTE values of monazite compounds seem to increase with the larger RE cations and RE-O distances in the structures.The CTE value is related to the inter-atomic potential and depends on the dissymmetry of the potential well. 34Examples of calculated CTE values for both monazites and xenotimes are shown in Fig. 8. 121 The C p values of monazite compounds are similar.The total heat capacity is the sum of the lattice component and an excess electronic term.The excess electronic heat capacity term increases for CePO  behavior of monazite compounds, and this brittleness could cause inaccuracies when using micro-indentation analysis.The CTE values of monazites increased with smaller RE cations and RE-O distances in the structures and were in the 10-11.5 × 10 −6 K −1 range.The heat capacities of all the synthesized monazites were ∼105-112 J mol −1 K −1 (with La and Gd having the lowest values).Controlling porosity during fabrication was important for achieving accurate measurements of both thermal and mechanical properties.Li et al. 121 calculated theoretical mechanical and thermal properties of monazite (La / Gd) and xenotime (Tb / Lu, Y, and Sc) compounds using the chemical bond theory of dielectric description.The CTE values increased with larger RE cations in the structures whereas the bulk moduli and lattice energies decreased.The CTE values were in the range of 7.78-7.47× 10 −6 K −1 for the La / Gd monazite compounds and 5.88-5.72 × 10 −6 K −1 range for Tb / Lu xenotime compounds.The CTE and bulk modulus values were dependent on the RE-O bonds.The RE-O distances were ionically dominated and changed with different RE cations due to lanthanide contraction, and PO 4 tetrahedra showed relatively high lattice energies and behaved nearly rigidly during deformation.
Kenges et al. 118 synthesized pellet samples of LaPO 4 monazite compounds with different sintering temperatures from 900 °C to 1500 °C and measured mechanical and thermal properties.The LaPO 4 compounds contained a small amount of impurity phase La(PO 3 ) 3 (lanthanum metaphosophate).The LaPO 4 monazite pellet sintered at 1100 °C showed the highest Young's modulus, toughness, and thermal conductivity.Increasing the sintering temperature increased the crystallite size and decreased the porosities of pellets.The LaPO 4 monazite pellet sintered at 1100 °C had a Young's modulus of 132 GPa, a hardness of 4.6 GPa, a toughness of 1.6 MPa m 1/2 , a CTE of 8.2 × 10 −6 K −1 , and a thermal conductivity of 3.2 W m −1 K −1 at 25 °C.
Deepthi and Balamurugan 131 compared the exural strength and Young's modulus of LaPO 4 and LaPO 4 mixed with 20 mass% Y 2 O 3 .The LaPO 4 /Y 2 O 3 pellets were sintered at 1000-1600 °C, and a pellet sintered at 1400 °C resulted in more uniform grain structure with less porosity.The LaPO 4 /Y 2 O 3 pellet showed a reduction in exural strength by 22% but increase of 1.05% in Young's modulus compared to the LaPO 4 pellet.
Popa and Konings 128 synthesized EuPO 4 and SmPO 4 monazite compounds and calculated their heat capacities.They used the enthalpy data of Sm and Eu monazites along with data from other La, Ce, Nd, and Gd monazites and calculated the heat capacity as a sum of lattice contributions and an excess electronic term.The calculated heat capacities of La / Gd monazites were in the range of 101.28-111.49J mol −1 K −1 range, and these values were in good agreement with the experimental values.
Hay et al. 93 investigated the phase transformations and deformation mechanisms of GdPO 4 , TbPO 4 , and DyPO 4 compounds using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) aer indentation.The synthesized powders of TbPO 4 and DyPO 4 had xenotime structures whereas GdPO 4 had a monazite structure.These materials were cold pressed at ∼300 MPa and sintered at 1600 °C for 20 h and 1700 °C for 1 h.The GdPO 4 , TbPO 4 , and DyPO 4 compounds showed Young's moduli of 199, 48, and 127 GPa and hardness values of 7.8, 1.3, and 4.6 GPa, respectively.The authors observed stress-induced transformations from xenotime to monazite and suggested ferroelastic behavior under certain conditions.The bulk modulus values were decreasing in order of Gd, Eu, Dy, and Tb compounds.Both Eu and Gd compounds showed hardnesses of 8-9 GPa whereas Tb and Dy compounds showed hardnesses of 6-7 GPa.Hardnesses for all four compounds were not impacted by indentation depth or strain rate.
In addition to experimental methods, rst principles calculations were also used to calculate the structural and physical properties of monazite and xenotime crystals.Kowalski and Li 120 calculated elastic moduli of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, and Dy monazites using ab initio density functional theory (DFT).The Margules interaction parameters, which is related to excess enthalpy of mixing in a RE1 x RE2 (1−x) PO 4 solid solution, and moduli were related to the mismatch in the endmember volumes of different RE cations within the structures.The computed Young's moduli, bulk moduli, and shear moduli increased with decreases in RE cation radii.The range of Young's moduli, bulk moduli, and shear moduli were 139-165 GPa, 110-129 GPa, and 54-65 GPa, respectively.Feng et al. 119 calculated theoretical mechanical and thermal properties of monazite compounds using the results of local spin density approximation and compared to the experimental values.The Young's moduli, bulk moduli, and shear moduli of LaPO 4 , CePO 4 , NdPO 4 , SmPO 4 , EuPO 4 , and GdPO 4 increased with smaller RE cations and shorter RE-O distances in the structures.The Young's modulus of monazite compounds showed high anisotropy.The calculated coefficients of linear thermal expansion were similar to experimental values, but the calculated thermal conductivities were higher than experimental values measured at >800 K.
Blanca-Romero et al. 132 used the DFT + U method to calculate the structures and thermodynamic properties of monazite type crystals, with a goal to test the accuracy of the method for modeling f electron-containing systems such as RE-monazites.They found signicant improvement both in terms of structures (lattice parameters, unit cell volumes, and RE-O distances for both RE oxides and phosphates) and properties (formation energies for RE phosphates and band gaps for RE oxides) of the DFT + U method as compared to the standard DFT in comparison to experimental values.Overall, the DFT + U method with the PBEsol (Perdew-Burke-Ernzerhof functional revised for solids) exchange correlation functional and Hubbard U parameters derived from linear response-based ab initio calculations was found to be a good choice for studying RE oxides and monazites.
Beridze et al. 133 further investigated the DFT + U method for ab initio calculations of xenotime-and actinide-bearing complexes.The accuracy of the description of RE-O bond distances in xenotime was compared for two standard DFT xc functionals (PBEsol with f electrons in the core and f electrons in the valence shells) and the DFT + U (PBEsol + U LR , with the U values calculated from ab initio linear response).It was found that the DFT + U method has the best description of RE-O distances in xenotime crystals while both DFT + U and DFT (PBEsol with f electrons in the core) describe formation energies with good agreement with experiment, when accounting for the overestimation of P 2 O 5 volume, similar to the case of monazites.
In a study by Teng et al., 140 a mixed Ce 0.5 Pr 0.5 PO 4 monazite sample set was produced from Pr 6 O 11 , Ce 2 (C 2 O 4 ) 3 $10H 2 O, and ADP.Particles were ball milled in ethanol (4 h), dried at 60 °C (18 h), and calcined at 1000 °C (2 h).Calcined products were mixed with 5-10% polyvinyl alcohol (PVA).The calcined material was cold uniaxially pressed at 60 MPa, cold isostatically pressed at 200 MPa, and then calcined at 500 °C (6 h).Finally, samples were hot-pressed in evacuated graphite molds (<10 Pa of atmosphere, 30 MPa applied load) at temperatures of 1050-1250 °C for 0.5-4 h.A select set of samples (1150 °C for 2 h) were subjected to MCC-1 (ASTM C1220) chemical durability tests at a 1 : 10 cm −1 surface-area-to-volume (sample : leachant) ratio at 90 °C in PTFE containers for different testing durations ranging from 3 d to 42 d where leachates were quantitatively measured using inductively coupled plasma mass spectrometry (ICP-MS).Aer 42 d of leaching, the NR i values were found to be 5.7 × 10 −6 g m −2 d −1 for Ce and 5.3 × 10 −5 g m −2 d −1 for Pr.
In a study by Oelkers and Poitrasson, 137 dissolution tests were run on natural monazite from Manangotry (Madagascar) in open mixed ow reactors under different conditions of 50-229 °C (pH = 2) and 70 °C (pH = 1.6, 2.6, or 10) and closed reactors at 70 °C (pH = 2, 6, or 10).The starting material was ground to a 50-100 mm particle size fraction.The results showed a range of leach rates with the highest releases revealed for the highest test temperatures (T200 °C).
In a study by Terra et al., 89 several monazites were made in the solid solution system of La (1−x) Gd x PO 4 (x = 0, 0.2, 0.3, 0.35, 0.4, 0.5, 0.6, 0.65, 0.7, 0.8, and 1.0).Starting materials included La(NO 3 , and H 3 PO 4 .An excess of H 3 PO 4 was included to prevent RE 2 O 3 formation during synthesis.Aqueous solutions of the RE reagents (i.e., 0.6-1.4M) were prepared and these were added to 5 M H 3 PO 4 .Three different methods were used to synthesize the compounds.In the rst approach, heat was applied in different 50% H 2 O vapor balanced with O 2 at 0.3 cm s −1 , 1500 °C, and 1 atm for 80 h.

× 10
a ASTM denotes American Society for Testing and Materials International; DIW denotes deionized water; MCC denotes Materials Characterization Center; NR i denotes the i-th element normalized release rate; PTFE denotes polytetrauoroethylene; RT denotes room temperature (i.e., ∼20-25 °C).b Natural monazite from Manangotry (Madagascar) listed with composition of (Ca 0.04 La 0.21 Ce 0.43 Pr 0.05 Nd 0.15 Sm 0.02 Gd 0.01 Th 0.13 )P 0.90 Si 0.09 O 4 with a calculated (normalized) molecular weight of 248.85 g mol −1 .steps: (1) samples were dried through direct evaporation using a sand bath, (2) dried material was ground to ner particle size and heated at 400 °C (14 h), and (3) heated at 1300 °C (10-14 h).In the second approach, the mixture was placed in a PTFE container at 150 °C in a sand bath for 1-2 weeks.The third approach utilized a hydrothermal synthesis process with additives in PTFE Parr autoclaves at 150-200 °C for 1-2 months.The GdPO 4 compound was subjected to chemical durability testing as shown in Table 7.
In a study by Du Fou de Kerdaniel, 136 AnPO 4 $xH 2 O compounds (An = Th, U) were produced using low-temperature methods based on procedures by Terra et al. 143 and Clavier et al. 144 or using dry chemistry routes by grinding mixtures and heating these mixtures to 1100-1400 °C where the target cations were Th, U, and REs.Leaching studies were performed under saturation in either static tests (in HDPE with low leachate renewal) or dynamic tests (in PTFE with high leachate removal) where solutions were analyzed with ICP-MS, time-resolved laser uorescence spectroscopy, or a-scintillation counting.Dissolved solids were determined as mass loss and reported as R L(i) (i = element of interest) where R L(i) values ranged from 10 −7 to 10 −3 g m −2 d −1 .
In a study by Hikichi et al., 126 the stabilities of RE elements of ErPO 4 , YbPO 4 , LuPO 4 , and YPO 4 xenotime compounds in acidic or basic aqueous environments were investigated.The mass% losses of RE elements from xenotime compounds were measured using HCl, H 2 SO 4 , HNO 3 , NaOH, NH 4 OH solutions with concentrations of 6-36 N, and the test was conducted at 20 °C for 30 d.The results showed that the tested xenotime compounds were stable in these solutions, and the mass losses of RE elements were < 0.7 mass%.
In a study by Han et al., 125 xenotime powders including ErPO 4 , YbPO 4 , LuPO 4 , ScPO 4 , and YPO 4 were cold pressed into pellets at 50 MPa and sintered at 1500 °C for 20 h in air.Chemical durability tests were performed using a vapor ow of 50% H 2 O balanced with O 2 at 0.3 cm s −1 at 1500 °C and 1 atm for 80 h.The results showed dissolution rates of 5-10 × 10 −2 g m −2 d −1 for xenotime compounds.However, the tests were performed in an alumina tube furnace, and formation of Al 5 RE 3 O 12 compounds were observed.
Rauddin and Grosvenor 145 investigated the room-temperature chemical durabilities of monazite-type, xenotime-type, and rhabdophane-type compounds on ne particles with specic surface areas ranging 1.3-15.1 m 2 g −1 .Two types of tests were run including dynamic tests over 84 d and static tests over 3 months.The tests revealed that the rhabdophane (GdPO 4 $H 2 O) structure released higher quantities of Gd and P ions in deionized water within the initial week of exposure.Analysis of the long-range and local structures of these materials indicates that the structures of these materials remained unchanged aer seven months of leaching.
Wronkiewicz et al. 146 studied the chemical durabilities of glass-crystal composites (GCCs) containing different crystalline phases, including apatite, monazite [(Ce,U)PO 4 ], and spinel.The tests run included the product consistency test (PCT, t = 7 d, 28 d, and 91 d), 147 the vapor hydration test (VHT), 148 and the Toxicity Characteristic Leach Procedure (TCLP). 149The monazite was noted as having fewer elements (i.e., Ce, U, Th) compared to the apatite sample.The leachate from the PCTs (t = 7-91 d) was noted as being slightly acidic (pH z 5.6-6.2).The apatite and monazite samples were noted as being 100-300× more durable than a glass waste form (i.e., SRL-202U) for U release.
Poitrasson et al. 150 studied naturally formed magmatic monazites from European-based Paleozoic granites.Their studies showed that the hydrothermal alteration of these monazites was notably complex including a variety of mechanisms such as monoclinic / hexagonal crystal structure transitions, chemical exchanges, cation substitutions, selective Th removal, dissolution followed by precipitation, and dissolution with replacement by different minerals.They also noted that temperatures up to 300 °C likely occurred within the vicinity of the minerals.
Mikhailova et al. 151 studied Pu-containing Eu-monazite and have documented Pu release being attributed to the formation of hydrated (rhabdophane) PuPO 4 by storage in air.This calls into question the long-term disposal potential for actinides stored in REPO 4 compounds if moisture is present in the atmosphere, which could be reduced if storage was performed in inert and dry conditions.

Radiation stability
The majority of naturally occurring monazites have been affected by radiation due to a decay of actinides within these minerals. 152This radiation has the potential to cause metamict alterations in the crystal structures of minerals, consequently elevating their solubility.In contrast to many radioactive minerals, monazite retains its crystalline structure under substantial cumulative radiation doses.4][155] Self-recovery of damaged structure was not observed in zircon minerals (isostructural to xenotime) with SiO 4 , but partial structural recovery was observed in xenotimes with PO 4 . 156-158Table 8 summarizes the critical amorphization dose of monazite and xenotime compounds.
Meldrum et al. 155 studied the effect of irradiation on monazite (LaPO 4 ), xenotime (ScPO 4 ), ZrSiO 4 , and ThSiO 4 using 800 keV Kr + ions.Recrystallization energies for the compounds were calculated to be 3.1-3.3eV for the silicates and 1-1.5 eV for the phosphates.Radiation damage was monitored as a function of temperature, and above 700 °C, the amorphization of ZrSiO 4 could not be induced as the recrystallization process was faster than damage accumulation.The critical temperature was calculated to be only 35 °C for LaPO 4 , and the monazite would not undergo phase decomposition at the tested conditions.In another study by the same group, 159 monazite could not be amorphized when exposed to 800 keV Kr + ions at temperatures surpassing 175 °C.On the contrary, zircon underwent amorphization at temperatures reaching up to 740 °C.It was found that materials with the zircon structure (i.e., ZrSiO 4 and ScPO 4 ) could be amorphized at slightly elevated temperatures compared to compounds with the monazite structure under equivalent irradiation conditions.
Seydoux-Guillaume et al. 165 studied the healing of radiation damage in natural monazite with annealing at 500-1200 °C.The natural monazite contained two domains with distorted lattice areas with a-dose of 2.5 × 10 16 a mg −1 accumulated since 474 Ma ago.From 500-900 °C, partial healing of the lattice occurred.At 900 °C aer 10 days, only one domain remained, and a well-crystallized lattice was observed.The same group performed structural analysis using X-ray diffraction (XRD), TEM, SEM, and electron probe microanalysis and showed that monazites are not metamict despite the old ages of samples ranging from 24 to 1928 Ma. 166 Bregiroux et al. 167 synthesized monazite powders containing plutonium(III), plutonium(IV) and americium(III).They examined the response of the monazite structure to a self-irradiation using XRD.The results revealed a total amorphization of the crystalline structure aer 300 days, reaching a cumulative dose of 1.65 × 10 25 a m −3 .
Picot et al. 168 explored the impact of Au 2+ and He + ion irradiation on monazite to simulate a-decay effects.The Au 2+ ionirradiation induced signicant alterations in the material properties.At a damage level of 6.7 dpa, monazite displayed an approximately 8.1% increase in volume, a 59% decrease in hardness, and complete structural amorphization.Conversely, no changes in the properties of these compounds were noted following He + ion implantation.Deschanels et al. 169 investigated a-induced swelling in monazite and zirconolite ceramics.It was found that the macroscopic swelling and amorphization of monazite relied on the type of irradiation.Monazite samples irradiated externally with Au became amorphous and exhibited a maximum swelling of 8%.In contrast, the swelling in samples doped with 238 Pu was signicantly smaller, at approximately 1%.
Radiuddin and Grosvenor 157 studied the structural stabilities of La 1−x Yb x PO 4 materials implanted with Au ions.The longand short-range order of La 1−x Yb x PO 4 (x = 0, 0.3, 0.7, 1.0) are inuenced by ion-implantation, indicating the materials are prone to structural damage.Interestingly, in certain members of the La 1−x Yb x PO 4 series (x = 0.7 and 1.0), partial recovery of the structure was observed following high-dose Au ion implantation.
Sadhasivam and Rajesh 38 studied the effect of g-irradiation on the NdPO 4 monazite compound using 60 Co g-cell source at a dose rate of 4.5 kGy h −1 .Defect center, ionization, and charge trapping did not occur during irradiation, and no signicant structural change was observed up to 150 kGy g dose.The high level of g dose did not affect the crystallinity and optical properties.
Rauddin et al. 163 evaluated irradiation effects on the ErPO 4 xenotime compound structure using high-energy dual ion-beam irradiation of 1.5 MeV Au 2+ and 160 keV He + .The xenotime structure was found to undergo amorphization at a lower Au 2+ ion-uence than the monazite structure.Moreover, subsequential He + ion-irradiation on the amorphized ErPO 4 samples did not lead to the structural restoration of xenotime.Simultaneous ion-irradiation of Au 2+ and He + prevented the amorphization of ErPO 4 as higher amounts of electronic energy was applied, similar to the a-healing mechanism in the monazite structures where high energies cause recrystallization faster than damage accumulation.However, the a-healing mechanism for xenotime required ∼4 times more energy compared to monazite.
Overstreet et al. 170 investigated the structural stability of SmPO 4 and TbPO 4 under swi heavy ion irradiation using 1.1 GeV 197 Au ions.Both SmPO 4 monazite and TbPO 4 xenotime structures experienced amorphization at comparable rates with increasing uence, and complete amorphization occurred ∼5 × 10 12 ions cm −2 for both compounds.No irradiation-induced recrystallization was observed at higher uences for both compounds.Findings from this study differed from other radiation damage using relatively low-energy ions, where the monazite compounds were more resistant to amorphization compared to xenotime compounds.The results from this study suggested that the crystal chemistries and structures of monazite and xenotime will not greatly affect the radiation tolerance to highly energetic ions.
Tisdale et al. 164 synthesized single crystals of Rb 3 RE(PO 4 ) 2 (RE = Y, La, Pr, Nd, and Sm/Lu) by high-temperature ux growth methods.The 1.2 MeV Xe 3+ ions were used on the Rb 3 Nd(PO 4 ) 2 sample to investigate the radiation effect, and complete amorphization was observed by 0.22 dpa for a single crystal sample and ∼0.52 dpa for a polycrystalline sample.DFT calculations were performed for trivalent actinide analogs of Rb 3 M(PO 4 ) 2 (M = Am, Cm), and the results indicated high tolerance to radiation damage.
Burakov et al. 161 studied radiation resistance effects, including amorphization, of different crystalline host phases on  24,25 An example of where this would have utility is where it is not practical to use metal thermocouples, e.g., remote sensors within a microwave environment.

Future work and perspectives
Aer considering the wealth of data collected on the wide range of anhydrous REPO 4 compounds discussed within this paper, more work is needed to ll in research gaps summarized within this section.Additional thoughts and perspectives are provided throughout where future work could be done.Regarding mechanical properties, extensive datasets are available for E, B, and G while several experimental gaps exist in datasets for H and FS (see Table 5).Many of the datasets from single studies do not correlate well with data from other studies or measured data fall far from calculated data (see Fig. 7), leading to difficulty in making accurate predictions in some cases.Gaps exist in the thermal properties for the different series (Table 6) and the data spread across literature values for single properties of a given REPO 4 compound tends to be rather high in some cases.For one dataset of calculated CTE values, the spread was very low (see Fig. 8), but that was anomalous against the entire set of summarized data.Two of the most populated datasets include C p and CTE.More work needs to be done to better elucidate the variabilities in these properties across the full RE dataset.
Since APO 4 compounds are promising as stable options for long-term disposal of radionuclides, including rare earths and actinides, more detailed studies on their chemical durabilities are needed.The normalized release rates (NR i ) for tested REPO 4 compounds show very low values (Table 7) as compared to other nuclear waste forms, such as borosilicate glass.Understanding potential incongruent elemental release from these compounds is important, including comparisons between NR P and NR RE over long time scales under dilute conditions.
While some radiation stability tests have been documented in the literature for REPO 4 compounds (see Table 8 for examples), additional studies are needed.This includes wider compositional ranges as well as more high-energy exposure studies (e.g., g-rays).Several studies have demonstrated amorphization doses for a variety of REPO 4 compounds and types of irradiations.A study by Nasdala et al. 179 provided evidence that a-assisted annealing can prevent irradiationinduced amorphization in CePO 4 monazite, but only above a specic damage level.Understanding how REPO 4 compounds behave in potential geological repository environments and the transition of REPO 4 compounds to the hydrated (i.e., rhabdophane or REPO 4 $xH 2 O) forms is very important from a waste form perspective for long-term disposal.
An overview of DFT-based rst principles calculations on mechanical, structural, and thermodynamic properties was briey covered due to the effectiveness of the method to predict the structures and properties of this class of materials.It was found that the usage of the DFT + U method 180,181 to treat f electrons in RE elements was essential to improve the description of both structures and properties.More work can be done to study defect formation energies and mixing of RE elements in REPO 4 compounds using rst principles methods.First principles calculations can also be used to study high-entropy monazite-based or xenotime-based ceramics formation and properties through mixing of various RE elements.In addition, classical molecular dynamics (MD) simulations can be used to study radiation effects and thermomechanical behaviors. 182ecent advances using machine learning potentials 183 based on rst principles calculations can help to alleviate the bottleneck of interatomic potential availability.However, extensive testing and validation of these new potentials will be needed.Combining these techniques, one can expect computational methods to provide valuable information regarding the distribution of RE elements within the crystal lattice, the role of dopants in controlling properties, radiation induced structural change and amorphization, and the mechanisms governing phase transformations under extreme conditions.
6][187] These models can be utilized to predict properties such as phase stability, mechanical behavior, and chemical reactivity under various/extreme conditions.Future efforts, including constructing a more consistent dataset including trace elements, developing high-throughput parallel simulation routines, and standardizing characterization methods, would greatly benet the implementation of machine learning in studying monazite and xenotime composition-structure-property relationships.By training models on experimental and/or simulation data, researchers can extrapolate insights beyond the limits of traditional analysis, guiding experimental design and hypothesis generation.

Summary and conclusions
The synthesis methods, crystal structures, and properties of anhydrous monazite and xenotime crystalline materials are summarized within this review.Monazite and xenotime compounds can be synthesized with a variety of different methods including ux-assisted, solid state, hydrothermal, aqueous, dehydration, and gel-based methods.For both monazite and xenotime structures, with larger and lighter RE cations in the crystal structure, the unit cell parameters (i.e., a, b, c) and volumes (V) increase linearly whereas the densities (r) decrease nonlinearly.Similar trends were observed for solid solution compounds containing mixed RE cations.Some solid solutions containing RE cations with large difference in sizes (e.g., La and Yb) showed the presence of both monazite and xenotime phases.For RE cations, the distortions of REO x polyhedra were greater in monazites when compared to xenotimes.Decreasing RE radii increased the Young's modulus, bulk modulus, and shear modulus, and this can be due to stronger interatomic bonding as the average RE-O distances decreased with smaller RE cations.Different modulus values from different studies are also affected by different sample preparation and sintering conditions for the pellets.In general, the CTE values of monazite compounds seem to increase with the larger RE cations and RE-O distances in the structures.The calculated CTE values usually followed the trends, but the experimental CTE values from different studies oen varied greatly, and it was difficult to create a general trendline.First-principles DFT calculations have been shown to be a reliable predictive method for both the structures and properties of monazite and xenotime crystals, although care was needed in the description of the f-electrons and the DFT + U method was found to be reliable and computationally feasible approach for this purpose.The chemical and radiation resistance of monazite and xenotime are similar to that of zircon, and thus the natural minerals are oen used in geochronology.Monazite compounds are generally more resistant to irradiation damage compared to the xenotime compounds.

Fig. 1
Fig. 1 Illustration showing the relationship of (a) xenotime and (b) monazite structures of REPO 4 to the RE crystal radii (r c ) from Shannon. 49LuPO 4 (ICSD 46792) and LaPO 4 (ICSD 46788) were used to create the (a) xenotime and (b) monazite unit cell structures, respectively.

Fig. 2
Fig. 2 Structures of (a-c) monazite and (d-f) xenotime compounds.(a) Nine-fold coordination of the RE cation, (b) the chain of REO 9 and PO 4 along the c axis, and (c) atomic arrangements of RE and P viewed from the [001] direction in the monazite structure.(d) Eight-fold coordination of the RE cation, (e) the chain of REO 8 and PO 4 along the c axis, and (f) atomic arrangements of RE and P viewed from the [001] direction in the xenotime structure.The figure was made using CIFs of ICSD 431743 and ICSD 46792 for LaPO 4 monazite and LuPO 4 xenotime, respectively.

Fig. 3
Fig. 3 Relationships of (a-c) unit cell parameters, (d) unit cell volumes, and (e) densities with respect to RE r c of monazite compounds.Where multiple values were given, averages and standard deviations are reported.

4 ,
PrPO 4 , NdPO 4 , SmPO 4 , and EuPO 4 monazites but not for LaPO 4 and GdPO 4 due to empty and half-lled electron shell congurations, respectively, and this results in similar heat capacities among CePO 4 / EuPO 4 , and slightly lower values for LaPO 4 and GdPO 4 . 34,117u et al.117 synthesized polycrystalline monazite compounds of LaPO 4 , CePO 4 , NdPO 4 , SmPO 4 , EuPO 4 , and GdPO 4 and pressed into pellets using a spark plasma sintering apparatus at 1350 °C and 40 MPa for 3 min.Young's modulus increased linearly from LaPO 4 to GdPO 4 as the RE r c values decreased.The specic heat values of all the monazites were similar at a given temperatures from 25 °C to 1000 °C and increased as the temperature increased except Gd monazite, which showed lower values at higher temperatures (>600 °C) compared to other monazites.The thermal conductivities of monazite compounds decreased as the temperature increased from 25 to 1000 °C, except EuPO 4 and GdPO 4 that showed increased thermal conductivities from 800 to 1000 °C.This is possibly due to the contribution of radiation transport effect, which becomes more important at higher temperatures.The thermal diffusivity values showed similar behavior as thermal conductivity with respect to temperature.Perrière et al. 34 investigated the structural dependence of the mechanical and thermal properties of monazite compounds including LaPO 4 , CePO 4 , PrPO 4 , NdPO 4 , SmPO 4 , EuPO 4 , and GdPO 4 .The synthesized monazite powders were pressed into pellets at 1500 °C and 65 MPa for 0.1-20 h.Both Young's modulus and shear modulus values increased with larger RE cations in the structure.The bending strengths of 94-122 MPa and fracture toughness around 1 MPa m 1/2 showed the brittle

Fig. 5
Fig. 5 Relationships of (a and b) unit cell parameters, (c) unit cell volumes, and (d) densities with respect to RE r c of xenotime compounds.Where multiple values were given, averages and standard deviations are reported.

Fig. 6
Fig.6(a) Structure-field map of monazite and xenotime compounds based on the ionic radius (r i ) of A (e.g., RE) and X (e.g., P) in AXO 4 compounds.107Reprinted with permission from Aldred.107Copyright 1984 American Chemical Society.(b) Modified Bastide diagram for ABX 4 compounds.Reprinted with permission from Hay et al. 115 Copyright 2013 Elsevier.

Fig. 7 (
Fig. 7 (a and c) Young's modulus and (b and d) bulk modulus values of (a and b) monazite and (c and d) xenotime compounds as functions of crystal radius of the RE cation (r c ).

4. 2
Chemical durability In general, the chemical durabilities of RE-phosphate and actinide-phosphate compounds documented in the literature are very high.Solubility product constants (K sp ) for hydrated actinide-phosphate (AnPO 4 $xH 2 O) and RE-phosphate (REPO 4 -$xH 2 O) compounds, which cover rhabdophane, monazite, and xenotime compounds, have been found within the range of

Fig. 8
Fig. 8 Summary of calculated CTE values for both monazite and xenotime REPO 4 crystals from Li et al. 121 as a function of RE r c .

Table 1
Summary of monazite and xenotime synthesis.La / Dy* are monazite compounds, and Tb / Lu + Y, Sc are xenotime compounds.For the synthesized compound, S and P denote single crystal and polycrystalline compound.The ratio represents the mass ratio of RE oxide to flux The molar ratio of REPO 4 to ux.The t d is the dwell time, "T c " represents the temperature that the mixture was slowly cooled to grow single crystals, and r c is the cooling rate.Adelstein et al.37prepared CePO 4 monazite by heating Ce rhabdophane at 800 °C for 1 h.However, a different study by Mesbah et al. 47 showed that heating Nd, Eu, Gd, and Dy rhabdophane compounds at 200-500 °C converted them to NdPO 4 , EuPO 4 , GdPO 4 , and DyPO 4 with tetragonal P3 1 21 space group, which is different symmetry from monazite or xenotime.Heuser et al. 41 synthesized TbPO 4 and DyPO 4 with monazite structures using rhabdophane of TbPO 4 $nH 2 O and DyPO 4 -$nH 2 O.The rhabdophane compounds were made using a similar precipitation method by Boakye et al. a .
axis (Fig.2e).Fig.2fshows the atomic arrangement of RE and P atoms projected down[001].With smaller RE cations in the xenotime structures compared to monazite, the D values of REO 8 polyhedra in xenotime structures are lower than those reported for monazite structures.The xenotime compounds have shorter RE-O distances (2.2-2.4Å)compared to monazites (2.4-2.6 Å) (Fig.4a).The P-O bond distances of tetrahedra in xenotime compounds are generally shorter than those in monazite, but the differences are not signicant.Xenotime has only one atomic position for the oxygen atom whereas monazite has four oxygen atomic positions.For both monazite and xenotime compounds, RE-P distances are dependent on the RE size, and xenotime has one RE-P distance along the chain whereas the monazite has two different RE-P distances.The RE-P distances of xenotime crystals are generally longer than those of monazite, and RE-P distances between RE 3+ and P 5+ cations in xenotime are generally <3 Å.The CN eff values of xenotime compounds are smaller, in the range of 7.8-8.0,and smaller than monazite as expected (Fig. . © 2024 The Author(s).Published by the Royal Society of Chemistry RSC Adv., 2024, 14, 18978-19000 | 18983 Review RSC Advances De Biasi et al.

Table 4
Monazite and xenotime compounds with mixed cations and their compositional effects.M and X denote monazite and xenotime structures, respectively

Table 6
Thermal properties of monazite and xenotime compounds including heat capacity (C p ), coefficient of thermal expansion (CTE), thermal conductivity (k), thermal diffusivity (a), and melting temperature (T m ).The measured and calculated temperatures are shown in the parentheses and * denotes values that were calculated

Table 7
Summary of chemical durability data for REPO 4 compounds a including the composition (if given), test conditions, normalized release rate (NR i ) for element "i" (if given), and the reference i (g m −2 d −1 ) Ref.